This application is related to Japanese application No. 2002-015867 filed on Jan. 24, 2002, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a display element and a production method therefor. More specifically, the invention relates to the structure of a seal member which can accurately combine a pair of substrates between which a liquid crystal layer or an organic EL layer is held, and to a method for combining the pair of substrates with the use of the seal member.
2. Description of the Related Art
Flat panel display devices employing liquid crystal display elements are generally used as display devices for mobile phones and PDAs (personal digital assistants).
Such a flat panel display device includes a pair of glass substrates combined in a predetermined spaced opposed relationship with the intervention of a looped seal member and a liquid crystal filled in a space defined by the looped seal member between the substrates. The seal member is typically composed of a thermosetting resin such as an epoxy resin.
In recent years, there has been an increasing demand for thickness reduction, weight reduction and strength enhancement of the mobile phones and the PDAs.
To this end, an attempt has been made to employ thin, light and strong plastic substrates instead of the glass substrates for the flat panel display device. The plastic substrates have practically been applied to liquid crystal display elements of TN (twisted nematic) type and STN (super twisted nematic) type.
In recent years, the flat panel display device has been required to have a higher speed responsiveness and a higher contrast.
To satisfy this requirement, switching elements such as TFTs (thin film transistors) or MIM (metal insulator metal) elements are provided on the substrate to drive the flat panel display device by an active matrix driving method.
In general, a high temperature process at not lower than 300xc2x0 C. is required for the provision of the switching elements (e.g., TFTs) on the substrate. Therefore, it is difficult to provide the TFTs on a plastic substrate having a low heat resistance. Where the switching elements (e.g., TFTs) are provided on the plastic substrate, a highly heat resistant plastic substrate should be employed.
To this end, one of the substrates on which the switching elements are provided is composed of a highly heat resistant plastic material, and the other substrate (a counter substrate or a color filter substrate) on which the switching elements are not provided is composed of an ordinary plastic material. Alternatively, one of the substrates on which the switching elements are provided is composed of glass, and the other substrate on which the switching elements are not provided is composed of a plastic material.
The higher speed responsiveness, the higher contrast, the thickness reduction, the weight reduction and the strength enhancement can be achieved by employing the aforesaid types of substrates in combination as the pair of substrates for the flat panel display device.
However, where the combination of the highly heat resistant plastic substrate and the ordinary plastic substrate or the combination of the glass substrate and the plastic substrate is employed as the pair of substrates, the substrates have significantly different linear expansion coefficients.
Since the display elements are highly microminiaturized and arranged at a higher density in the recent flat panel display device, an electrode pattern provided on the one substrate and a color filter pattern provided on the other substrate should accurately be opposed in a predetermined positional relationship when the substrates are combined. Accordingly, very precise positioning of the pair of substrates is required when the substrates are combined.
Where the highly heat resistant plastic substrate and the ordinary plastic substrate are combined with the intervention of the thermosetting resin seal member and heated for curing the seal member, the ordinary plastic substrate expands to a greater extent than the highly heat resistant plastic substrate.
Where the glass substrate and the plastic substrate are combined with the intervention of the thermosetting resin seal member and heated for curing the seal member, the plastic substrate expands to a greater extent than the glass substrate.
If the other substrate expands to a greater extent than the one substrate and is fixed in an offset relationship with respect to the one substrate, the electrode pattern on the one substrate and the color filter pattern on the other substrate are not opposed in the predetermined positional relationship.
One known approach to this problem is to employ a seal member composed of a UV curable resin. Another known approach is to temporarily fix the substrates with their margins bonded by a seal member of a UV curable resin and finally bond the substrates with a seal member of a thermosetting resin (see, for example, Japanese Unexamined Patent Publication No. 2000-241821).
Where the pair of substrates are combined with the use of the UV curable resin seal member alone, there is a possibility that the substrates are offset from each other before the seal member is cured. This is because the UV curable resin seal member has a lower viscosity and a poorer adhesiveness to the substrates than the thermosetting resin seal member.
Where the pair of substrates are temporarily fixed with the use of the UV curable resin seal member and finally bonded with the thermosetting resin seal member, the less heat resistant substrate may be deformed to be bulged by heat applied for curing the thermosetting resin seal member.
As shown in FIG. 10, when a plastic substrate 101 is heated with its margin temporarily fixed by a seal member 107a of a UV curable resin, the plastic substrate 101 expands. At this time, the plastic substrate 101 cannot be displaced parallel to a glass substrate 102, but deforms perpendicularly to the glass substrate 102.
Since spacers are provided between the glass substrate 102 and the plastic substrate 101, the plastic substrate 101 deforms only in one direction (perpendicularly apart from the glass substrate 102). As a result, the plastic substrate 101 is bulged.
The glass substrate 102 and the plastic substrate 101 are fixed by a seal member 107b of a thermosetting resin with the plastic substrate 101 being in a bulged state. As a result, a spacing between the glass substrate 102 and the plastic substrate 101 increases in comparison with a predetermined spacing, as a distance from the UV curable resin seal member 107a increases. Therefore, the inter-substrate spacing cannot be kept uniform. This adversely affects the display quality of a display element 110.
Furthermore, the seal member 107b of the thermosetting resin often produces a gas component when it is cured. Where the gas component intrudes into a space defined by the seal member 107b, there is possibility that the display quality of the display element 110 is degraded.
In view of the foregoing, the present invention is directed to a display element and a production method therefor, by which a pair of substrates can be combined in a predetermined positional relationship at a predetermined spacing even if the substrates have significantly different linear expansion coefficients.
According to the present invention, there is provided a display element, which comprises a pair of substrates; a display layer provided between the substrates for performing a display operation; and a looped seal member provided between the substrates for enclosing the display layer between the substrates; the substrates having different linear expansion coefficients; the looped seal member comprising a plurality of resin layers stacked from an inner side to an outer side, at least one of the resin layers being composed of a thermosetting resin, the rest of the resin layers being composed of a UV curable resin.
In the present invention, the UV curable resin layer for temporarily fixing the pair of substrates and the thermosetting resin layer for finally fixing the pair of substrates are integrated into the seal member. Therefore, there is no spacing between the UV curable resin layer and the thermosetting resin layer, unlike the prior art in which the UV curable resin seal member and the thermosetting resin seal member are provided in a spaced relationship.
Even if the substrates to be combined have different linear expansion coefficients, there is no possibility that the substrates are finally fixed in a positional relationship different from an initial positional relationship of the substrates temporarily fixed by the UV curable resin seal member or finally fixed at a greater spacing than the predetermined spacing as in the prior art.
Since the UV curable resin layer and the thermosetting resin layer are not spaced from each other, the substrates are finally fixed in the same positional relationship as when the substrates are temporarily fixed at the predetermined spacing.